S-waves attenuation and separation of scattering and intrinsic absorption of seismic energy in southeastern Sicily (Italy)

Abstract

earthquake source parameters in a given region requires a good knowledge of attenuation and properties of the medium. Especially, information on high-frequency seismicwave attenuation in the lithosphere is of particular interest (Yoshimoto et al. 1993). In southeastern Sicily, the attenuation of seismicwaves has been studied by using different data set and methods (e.g. Castro et al. 1993; Giampiccolo et al. 2002, 2003, 2004; de Lorenzo et al. 2004). Castro et al. (1993) first obtained a frequency-dependent quality factor of coda waves (QC) and calculated the average attenuation law for the area. Giampiccolo et al. (2002, 2004) investigated in more detail both the frequency and lapse time dependence of QC by using different data sets. Their results show a good agreement with those obtained by Castro et al. (1993). Moreover, a clear increase of QC with lapse time and, therefore, with depth was observed. Giampiccolo et al. (2003) and de Lorenzo et al. (2004) estimated the quality factor of P-waves (QP) by applying the pulse-broadening method (Wu & Lees 1996) whereas Giampiccolo et al. (2003) estimated the attenuation of S waves (Q−1 S ) by applying the frequency decay method (see Bianco et al. 1999) in the low- (below the corner frequency) and high- (above the corner frequency and below the cut-off filter) frequency ranges. The obtained results suggested that attenuation at higher frequencies is less pronounced than at lower ones. However, the detailed frequency-dependent nature of QP and QS was not resolved yet by the above quoted studies. It isworth stressing that attenuation estimated from direct S waves contains the combined effects of scattering and intrinsic loss. Scattering attenuation is described by the quality factor Q−1 s and is due to the presence of inhomogeneities. Therefore, it depends on the spatial structure of the heterogeneities in the medium and on the size of the velocity and density fluctuations. Intrinsic absorption Q−1 i is caused by the anelasticity of the medium and depends on viscous processes between the rock matrix and liquid inclusions, such as pore fluids, and on movements of dislocations through the mineral grains (Goric & Muller 1987). Quantifying the relative contribution of scattering and intrinsic attenuation has been a subject of considerable interest among seismologists and different methods have been developed (e.g. Wu 1985; Hoshiba et al. 1991; Wennerberg 1993). In the present paper we will estimate the quality factor of S waves (QS) in the lithosphere beneath southeastern Sicily and clarify its frequency dependence by means of the coda-normalization method (Aki 1980), applied in the frequency range 1.5–15 Hz.We will also obtain a separate estimate of intrinsic and scattering attenuation by applying the multiple lapse time window analysis (MLTWA) technique by Hoshiba et al. (1991). This method gives information about the temporal change of seismic energy during a wave’s propagation by considering the energy in multiple consecutive time windows as a function of the hypocentral distance. Under the assumptions of multiple and isotropic scattering and uniformdistribution of scatterers, two attenuation parameters will be calculated: the seismic albedo B0, defined as the dimensionless ratio of the scattering loss to total attenuation (B0 = Q−1 s /Q−1 T ) and the inverse of the extinction length L−1 e that is the inverse of the distance (in kilometres) over which the primary S-wave energy is decreased by e−1. B0 ranges between 0 and 1 and was proposed byWu (1985) to describe the proportions of energy loss dominated by intrinsic attenuation (B0 < 0.5) or scattering attenuation (B0 > 0.5). The estimated scattering (Q−1 s ) and intrinsic (Q−1 i ) attenuation mechanisms in the frequency range 1.5–15 Hz will be discussed and compared with previous results obtained by Giampiccolo et al. (2004). Estimates of total attenuation Q−1 T will be compared with the coda-Q values (QCobs) obtained by Giampiccolo et al. (2004) with the expected coda-Q (QCexp) calculated in this study by using Hoshiba (1991) relationship. Finally, since the MLTWA technique has been widely applied to several areas in the world (e.g. Mayeda et al. 1992; Hoshiba 1993; Akinci et al. 1995; Pujades et al. 1997; Akinci & Eydo˘gan 2000; Bianco et al. 2002; Ugalde et al. 2002; Bianco et al. 2005), differences and analogies observed among southeastern Sicily and other tectonic settings will be discussed.